Many types of electrochemical devices, for example, lithium secondary batteries, electrolytic condensers, electric double-layer capacitors, electrochromic display devices and dye-sensitized solar cells, have been developed and are currently in use. Particularly, various researches are being done to find future practical uses of dye-sensitized solar cells. Such electrochemical devices employ various kinds of electrolytes, which are gaining more importance day by day.
Non-aqueous electrolyte solutions are presently the most widely used electrolytes. A typical non-aqueous electrolyte includes an ionizable salt such as a lithium salt, dissolved in a suitable organic solvent, such as ethylene carbonate, propylene carbonate, dimethoxyethane, γ-butyrolactone (GBL), N,N-dimethylformamide, tetrahydrofuran or acetonitrile.
However, organic solvents used in such non-aqueous electrolytes are prone to leakage due to their low viscosity and have a strong tendency to volatilize, posing a risk of evaporation. The organic solvents are highly flammable. These problems make electrochemical devices including the non-aqueous electrolytes less durable and stable.
In an effort to solve the above problems, imidazolium-based and ammonium-based ionic liquids have been proposed as electrolytes for lithium secondary batteries. However, the ionic liquids are reduced at higher voltages than the lithium ions at the anodes, or the imidazolium and ammonium cations, together with the lithium ions, are intercalated into the anodes, thus deteriorating the performance of the batteries.
Korean Patent No. 10-751203 and Korean Unexamined Patent Publication No. 10-2007-85575 disclose eutectic mixtures as electrolytes, each of which includes an amide compound and a lithium salt, wherein the amide compound is acetamide, urea, methylurea, caprolactam, valerolactam, trifluoroacetamide, carbamate, formamide or the like represented by the corresponding formula. The eutectic mixtures have relatively wide electrochemical windows and exhibit good thermal and chemical stability. Due to these advantages, the eutectic mixtures are protected from evaporating and catching fire, which are problems of conventional electrolytes arising from the use of organic solvents.
As a result, rapid development of various electrolytes is actively underway. Particularly, there is a growing need for electrolytes that contain compounds with higher thermal stability and wider electrochemical windows so as to be applicable to electrochemical devices requiring various electrochemical properties.